Vehicle Transmission With Clutch Pack Overrun

ABSTRACT

A drive system ( 14, 34 ) comprising an input shaft ( 112 ); a first clutch ( 114 ) configured to selectively couple a first gear ( 116 ) to the input shaft ( 112 ); an output shaft ( 126 ); a second gear ( 138 ) engaging with the first gear ( 116 ); and a drive axle clutch system ( 142 ) having different travel direction clutches.

RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent ApplicationSer. No. 61/156,042 filed on Feb. 27, 2009, to PCT application Ser. No.PCT/US10/25408 filed on Feb. 25, 2010, and to co-pending U.S. patentapplication Ser. No. 13/202,173 (published as U.S. Pat. Pub. No.2011/0303505 A1 on Dec. 15, 2011) which are all herein incorporated byreference in their entirety.

BACKGROUND

A torque converter is used for transferring rotating power from a primemover, such as an internal combustion engine or electric motor, to arotating driven load, such as a vehicle Like a basic fluid coupling, thetorque converter normally takes the place of a mechanical clutch,allowing the load to be separated from the power source.

The torque converter has three stages of operation. During a stallstage, the engine is applying power to a torque converter pump but atorque converter turbine cannot rotate. For example, in an automobile,this would occur when the driver has placed the transmission in gear butprevents the vehicle from moving by continuing to apply the brakes.During an acceleration stage, the vehicle is accelerating but therestill is a relatively large difference between pump and turbine speed.During a coupling stage when the vehicle is moving, the turbine reachesa larger percent of the speed of the pump.

The torque converter is used for smoothing the engagement of the engineto the drive train. However, torque converters are generally inefficientand much of the wasted energy is expended in the form of heat. Forexample, there is zero efficiency during the stall stage, efficiencygenerally increases during the acceleration phase, and it is stillmoderately inefficient during the—coupling stage.

SUMMARY

A multi-speed transmission system replaces a torque converter bycontrolled clutch slipping. The multi-speed transmission is alsodesigned to replace the torque amplification normally provided by torqueconverters at low speeds. The transmission system uses one-way bearingsthat provide smooth transitions between gears and significantly improvethe efficiency of the transmission to the equivalent of a manualtransmission while eliminating the drag normally associated withhydraulic clutch packs.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a multi-speed transmission system.

FIG. 2 is a schematic cut-away view of a one-way bearing used in thetransmission system of FIG. 1.

FIG. 3A-3C are isolated views of a portion of the transmission systemshown in FIG. 1.

FIG. 4 is a block diagram of a control system used in conjunction withthe transmission system of FIG. 1.

FIG. 5 is a flow diagram showing in more detail how the transmissionsystem in

FIG. 1 operates when the vehicle is in a stopped position.

FIG. 6 is a state diagram showing in more detail how the transmissionsystem in FIG. 1 shifts between gears.

FIG. 7 is a flow diagram showing in more detail how the transmissionsystem in FIG. 1 operates when the vehicle is in a stopped position on agrade.

DETAILED DESCRIPTION

FIG. 1 is a schematic of a portion of a vehicle 10 that includes amulti-speed transmission system 14. The transmission system 14 uses aclutch pack overrun system 110 that eliminates some of theinefficiencies associated with torque converters. The vehicle 10 in oneembodiment is an industrial lift truck. However, the transmission system14 can be used in a variety of different vehicles.

The vehicle 10 includes an engine 12 that is connected to a drive axleassembly 34 through the transmission system 14. The engine 12 rotates aninput shaft 112 that then through clutch pack overrun system 110selectively applies torque and rotates an output shaft 126. The outputshaft 126 couples the transmission system 14 with the drive axleassembly 34 and causes the drive axle assembly 34 to rotate wheels 39.

One embodiment of the drive axle assembly 34 is conventional. In anotherembodiment, the drive axle assembly 34 uses a drive axle clutch system142 that includes different travel direction hydraulic clutches andgears to rotate wheels 39 in different directions and move the vehicle10 in different forward, reverse, and turning directions. The drive axleclutch system 142 is described in U.S. Provisional Patent ApplicationSer. No. 61/156,042 filed on Feb. 27, 2009, and U.S. patent applicationSer. No. 13/202,165 which have both been incorporated by reference intheir entirety.

It should be understood that the transmission system 14 can operate withany conventional axle assembly and vehicle direction control system. Thetransmission system 14 is not required to be used in conjunction withthe drive axle clutch system 142 described above, and can operateindependently of the drive axle clutch system 142. However, at least oneembodiment below describes how the transmission system 14 operates inconjunction with drive axle clutch system 142.

The transmission system 14 includes a first drive gear 116 selectivelyconnected to the input shaft 112 through a first hydraulic clutch pack114. A second drive gear 120 is selectively connected to the input shaft112 through a second hydraulic clutch pack 118 and a third drive gear122 is rigidly connected to the input shaft 112.

A first driven gear 138 engages with first drive gear 116 and engageswith the output shaft 126 through a first one-way bearing 136. A seconddriven gear 134 engages with a second drive gear 120 and is engaged withthe output shaft 126 through a second one-way bearing 132. A thirddriven gear 128 engages with a third drive gear 122 and is selectivelyconnected to the output shaft 126 by a third hydraulic clutch pack 130.

Hydraulic clutches 114, 118, and 130 operate similar to hydro-mechanicalclutches in power shift transmissions. The hydraulic clutches 114 and118 can selectively lock the gears 116 and 120, respectively, to theinput shaft 112 when rotating. The hydraulic clutch 130 can selectivelylock the gear 128 to the output shaft 126. Each hydraulic clutch isprovided with a proportional electro-hydraulic valve and hydraulicpressure sensor to provide for control and feedback (see FIG. 4).Alternative sensors, such as torque sensors can be used in place ofpressure sensors for closed feedback loop control.

Torque is transferred from input shaft 112 to output shaft 126 whenfirst hydraulic clutch 114 locks first drive gear 116 to input shaft 112and first one-way bearing 136 locks first driven gear 138 to outputshaft 126. Torque is also transferred from input shaft 112 to outputshaft 126 when second hydraulic clutch 118 locks second drive gear 120to input shaft 112 and second one-way bearing 132 locks second drivengear 134 to output shaft 126. Torque is also transferred from inputshaft 112 to output shaft 126 when third hydraulic clutch 130 locksthird driven gear 128 to output shaft 126.

The one-way bearings 136 and 132 lock the gears 138 and 134,respectively, to the output shaft 126 when turning in only one directionof shaft rotation. The one-way bearings 136 and 132 allow the outputshaft 126 to free wheel inside the driven gears 138 and 134,respectively, if the output shaft 126 turns faster than the driven gear.

FIG. 2 is a simplified sectional view showing some of the elements inone of the one-way bearings 136 or 132. FIG. 2 uses first one-waybearing 136 as an example. The first one-way bearing 136 is coupled tothe output shaft 126 and includes bearings 150 that press against aninside wall of the driven gear 138.

When the first driven gear 138 has a rotational speed 152 that is fasterthan the rotational speed 154 of output shaft 126, the first one-waybearing 136 automatically locks the first driven gear 138 to the outputshaft 126. The first one-way bearing 136 automatically releases thefirst driven gear 138 from the output shaft 126 when the output shaft126 starts rotating at a faster rotational speed 152 than the firstdriven gear 138. This unlocked one-way bearing state is alternativelyreferred to as free-wheeling.

When the first driven gear 138 is overrun by the output shaft 126, thefirst drive gear 116 in FIG. 1 cannot transfer torque from the inputshaft 112 to the output shaft 126. There is also very low drag when thefirst one-way bearing 136 is in the unlocked free-wheeling state. Theone-way bearing is used to accomplish an up or down shift. Again, itshould be noted that FIG. 2 is a simplified representation of a one-waybearing, and other one-way bearing configurations can also be used intransmission system 14.

FIGS. 3A-3C describe in more detail how the transmission system 14operates. The rotational states 160A-160H refer to different rotationalstates of the shafts 112 and 126 and different rotational or lockingstates of the one-way bearings and hydraulic clutches.

Referring first to FIG. 3A, the hydraulic clutch 114 is activated andhydraulic clutches 118 and 130 are deactivated. Plates in the firsthydraulic clutch 114 press together in the active state, coupling thefirst driven gear 116 to the input shaft 112. Activating first hydraulicclutch 114 causes a rotation 160B in first drive gear 116. Depending onthe current state of the vehicle 10 either in a stopped or movingcondition, the first hydraulic clutch 114 may be slipped to graduallyengage the input shaft 112 with first drive gear 116 or the firsthydraulic clutch 114 may be locked.

The first drive gear 116 has a relatively low rotational speed 160B,creating a rotational speed 160C in first driven gear 138. However,output shaft 126 is currently not rotating and the faster rotation 160Cof first driven gear 138 causes the first one-way bearing 136 to lockfirst driven gear 138 to output shaft 126. The locking of first one-waybearing 136 allows the first drive gear 116 to apply torque to theoutput shaft 126 and start output shaft 126 rotating with a rotationalspeed 160H.

The second hydraulic clutch 118 is currently not activated, so thesecond drive gear 120 is unlocked and has no rotational speed 160D andthe second driven gear 134 is unlocked and has no rotational speed 160E.Since the output shaft 126 is rotating faster than stationary seconddriven gear 134, the second one-way bearing 132 does not engage and theoutput shaft 126 freewheels inside of the second driven gear 134. Inthis stage, the second drive gear 120 does not apply any torque to theoutput shaft 126.

The third drive gear 122 is permanently attached to the input shaft 112and has a rotational speed 160F and limited torque that rotates thethird driven gear 128. However, the third hydraulic clutch 130 iscurrently not activated and therefore the third drive gear 122 also doesnot apply torque to the output shaft 126.

The high gear ratio of driven gear 138 to drive gear 116 provides hightorque to the output shaft 126 for pushing. The first hydraulic clutch114 can also be used as an inching clutch for starting and finepositioning. However, any of the other clutches may be also used forinching control.

FIG. 3B shows how the transmission system 14 operates during atransition from first drive gear 116 to the second drive gear 120. Thesecond hydraulic clutch 118 is activated causing input shaft 112 torotate second drive gear 120 with rotational speed 160D. In thisexample, the first hydraulic clutch 114 is shown still activated and thethird hydraulic clutch 130 is still not activated. However, the firsthydraulic clutch 114 may be released sometime after the second hydraulicclutch 118 is activated.

The second drive gear 120 rotates the second driven gear 134 faster thanthe first drive gear 116 rotates the first driven gear 138 and outputshaft 126. Accordingly, the second one-way bearing 132 locks the seconddriven gear 134 to output shaft 126 and the second drive gear 120 startsapplying torque and a rotational speed 160H to the output shaft 126.Output shaft 126 is now rotating faster than the first driven gear 138causing the first one-way bearing 136 to release the first driven gear138 from output shaft 128. Output shaft 128 then starts free-wheelinginside of the first driven gear 138 and the first drive gear 116 nolonger applies torque to the output shaft 126. The third hydraulicclutch 130 is still deactivated and the third drive gear 122 still doesnot apply torque to the output shaft 126.

One advantage of the transmission system 14 is the simple relativelysmooth transitions between different gears. For example, the firstone-way bearing 136 automatically disengages when the second one-waybearing 132 engages. Thus, the disengagement of the first hydraulicclutch 114 does not have to be precisely coordinated with the engagementof the second hydraulic clutch 118. The use of a high gear ratio withgears 116 and 138 also eliminates the need for a torque converter. Theengine 12 (FIG. 1) also does not need to be revved up as high to preventstalling when transitioning to lower gear ratios.

FIG. 3C shows how the transmission system 14 operates during anothertransition from second drive gear 120 to the third drive gear 122. Thethird hydraulic clutch 130 is activated locking the third driven gear128 to output shaft 126. The third drive gear 122 has a rotational speed160F and applies torque and rotates the output shaft 126. In thisexample, the second hydraulic clutch 118 is shown still activated andfirst hydraulic clutch 114 is shown deactivated. However, anycombination of the hydraulic clutches 114 and 118 may be released or notreleased after hydraulic clutch 130 is activated. For example, it ispossible for all three hydraulic clutches to be engaged without gearsbinding.

The third drive gear 122 generates a rotational speed 160G in the thirddriven gear 128 which in turn creates a rotational speed 160H in theoutput shaft 126. In this embodiment the gear ratio between the thirddriven gear 128 and the third drive gear 122 is lower than the gearratio between the second driven gear 134 and the second drive gear 120.The gear ratio between the second driven gear 134 and the second drivegear 120 is lower than the gear ratio between the first driven gear 138and the first drive gear 116. Thus, the rotational speed 160H will befaster than both the rotational speed of the first driven gear 138 andfaster than the rotational speed of the second driven gear 134.Accordingly, the second one-way bearing 132 disengages the second drivengear 134 from output shaft 126 and the first one-way bearing 136 keepsthe first driven gear 138 disengaged from output shaft 126. Thus, thetransmission system 14 moves into third drive gear 122 without having tomechanically coordinate the disengagement of the other gears 116 and120.

A reverse process is used to downshift from the drive gear 122 back downto gears 120 or 116. Conventional transmission systems have tosimultaneously modulate both the deactivation of one gear clutch and theactivation of another gear clutch requiring a high degree ofcoordination to achieve smooth shifting. However, in the transmissionsystem 14, different hydraulic clutches can remain engaged duringupshifting and downshifting operations because of the overrunningcapability of the associated one-way bearings. As the transmissionsystem 14 shifts, one gear starts to transmit torque and stopsoverrunning as another gear is disengaged.

For example, the second hydraulic clutch 118 can be engaged while thethird hydraulic clutch 130 is disengaged. This allows the second drivegear 120 to eventually start rotating the second driven gear 134 fasterthan the output shaft 126. The second one-way bearing 132 then engagesthe second driven gear 134 with the output shaft 126 and allows thesecond drive gear 120 to start applying torque to the output shaft 126.

Similarly, the first hydraulic clutch 114 can be engaged while thesecond hydraulic clutch 118 is disengaged. The first one-way bearing 136locks the first driven gear 138 with output shaft 126 when therotational speed of the first driven gear 138 overtakes the rotationalspeed of output shaft 126. The first drive gear 116 then starts applyingtorque to the output shaft 126.

There is a relatively smooth transition from the third drive gear 122 tothe second drive gear 120 and from the second drive gear 120 to thefirst drive gear 116. This is due to the one-way bearings 132 and 136only locking the output shaft 126 with driven gears 134 and 138,respectively, when the speed of the driven gears overtake the rotationalspeed of output shaft 126. Thus, the vehicle jerking that normallyoccurs in conventional transmission systems when transitioning betweengears may be reduced.

During “free wheeling” when going down a grade in first drive gear 116,the output shaft 126 may overrun the first driven gear 138. One controlstrategy is to shift to third drive gear 122 and let third driven gear128 provide some degree of engine braking. The clutch system 142 locatedin the drive axle assembly 34 in FIG. 1 can also be used for braking thevehicle 10.

Control System

FIG. 4 shows a control system for the vehicle 10 and transmission system14 previously shown in FIGS. 1-3C. A Central Processing Unit (CPU) 40controls the activation of hydraulic clutch packs 114, 118, and 130 inthe transmission system 14 according to different vehicle parameters. Acontrol valve 16 in the transmission 14 controls fluid pressure thatcontrols the activation of the clutch packs 114, 118, and 130.

The CPU 40 receives a vehicle speed and direction signal 18 from avehicle speed sensor 200 that indicates the Transmission Output Shaftrotational Speed (TOSS) and direction of the output shaft 126. An EngineRotations Per Minute (ERPM) signal 30 is generated from an engine speedsensor 204 and indicates how fast the input shaft 112 (FIG. 1) connectedto the engine 12 is rotating. An engine governor control signal 32controls a throttle valve 206 that controls the speed of engine 12. Atransmission temperature signal 28 is generated by a temperature sensor208 and identifies the temperature of the transmission fluid in thetransmission 14.

The CPU 40 receives a brake pedal position signal 42 from a brake pedalposition sensor 210 on brake pedal 43. An accelerator pedal positionsignal 44 is received from an accelerator pedal position sensor 212 onaccelerator pedal 50. The accelerator pedal position can alternativelycorrespond to a throttle value, acceleration value, or decelerationvalue.

A forward-reverse direction signal 46 is generated by a direction leveror pedal 52 and indicates a forward or backward direction the vehicleoperator selects for the vehicle 10. An internal or external memory 48contains mapped parameters identifying clutch pressure values and othercontrol and speed parameters used for performing different braking andshifting operations. Some of the parameters stored in memory 48 aredescribed in more detail below in FIGS. 5-7.

The hydraulic clutches 114, 118, and 130, in combination with one-waybearings 136 and 132 selectively engage and disengage the input shaft112 with the output shaft 126 as described above. The engaging force ofthe hydraulic clutches 114, 118, and 130 are controlled by changing theoil pressure in the clutch chambers. The oil pressure in the clutchchambers is controlled by the control value 16 which is controlled bythe CPU 40.

Control valve clutch signal 22 controls the oil pressure in the firsthydraulic clutch pack 114, control valve signal 24 controls the oilpressure in the second hydraulic clutch pack 118, and control valvesignal 26 controls the oil pressure in the third hydraulic clutch pack130. Where the drive axle clutch system 142 in FIG. 1 is used, one ormore signals 70 control the oil pressure(s) for the clutch system 142(FIG. 1) in the drive axle assembly 34.

Pressure sensor signal 56 indicates the amount of pressure applied bythe control valve 16 in the hydraulic clutch pack 114. Pressure sensorsignal 60 indicates the amount of pressure applied in the hydraulicclutch pack 118 and pressure sensor signal 64 indicates the amount ofpressure applied by the control valve 16 to the hydraulic clutch pack130. When hydraulic clutch packs are used in the drive axle 34, one ormore pressure sensor signals 72 indicate the amount of pressure appliedto the hydraulic clutch packs 142. When a conventional drive axle isused, pressure sensor signal 72 is not used.

The CPU 40 uses the signals 56, 60, and 64 to determine the amount ofslipping in the hydraulic clutch packs 114, 118, and 130, respectively.When any of the clutch pressures are zero, the particular hydraulicclutch 114, 118, or 130 disengages that associated gear from the inputshaft 112 or output shaft 126. When the clutch pressure for any of thehydraulic clutch packs is at a maximum pressure, the correspondingclutch pack maximizes the engaging force between the associated shaftand gear (locking). When the clutch pack pressure is between zero andthe maximum value, the corresponding clutch pack is partially engaged.The partially engaged condition is referred to as “clutch packslipping.”

As mentioned above, the drive axle 34 can be a conventional drive axlethat does not use hydraulic clutch packs. However, when located in thedrive axle assembly 34, the clutch system 142 permits the application oftorque from the engine 12 to be separated from clutch pack braking. Thispermits engine speed control independent of ground speed. For example,an operator may wish to speed up the engine 12 for hydraulic operationswhile decreasing the vehicle travel speed. This can be performedautomatically by having the CPU 40 disengage the transmission 14 andapply clutch pack braking in the drive axle assembly 34.

FIG. 5 is a flow diagram describing one way the control system in FIG. 4operates when the vehicle 10 is stopped in state 300. In operation 302,the CPU 40 receives a command to move the vehicle 10. For example, theCPU 40 may receive the accelerator pedal position signal 44 responsiveto the Accelerator Pedal Position (APP) of accelerator pedal 50. Inoperation 304, the first hydraulic clutch 114 is slipped by the CPU 40by controlling the amount of pressure supplied by control valve 16 viasignal 22. The slipping of the first hydraulic clutch 114 limits torque,preventing engine 12 from stalling, and reduces drive gear engagementshock to the drive axle assembly 34 and the vehicle operator. Thisclutch pack slipping replaces at least one of the functions of a torqueconverter, namely preventing the engine 12 from stalling when thevehicle 10 starts moving from a stopped position.

Selected clutches in clutch system 142 when used in the drive axle 34are also engaged in operation 304 according to the selected traveldirection and slope of the vehicle 10. For example, a first set ofclutches in clutch system 142 may be selected for engagement by CPU 40via signals 70 to move the vehicle 10 in a forward direction and asecond set of clutches in clutch system 142 may be selected forengagement by the CPU 40 to move the vehicle 10 in the reversedirection. The direction of the vehicle 10 may be determined by the CPU40 via the direction sensor signal 46.

In operation 306, the CPU 40 continues to increase the pressure suppliedby control valve 16 to the first hydraulic clutch 114 andcorrespondingly increases the amount of torque supplied by the engine 12to the drive axle 34 according to operator intent. For example, the CPU40 continuously monitors the position of accelerator pedal 50 todetermine how much pressure and associated slipping to apply in thefirst hydraulic clutch pack 114 using signal 22 and to determine whatspeed to run engine 12 using signal 32.

The CPU 40 in operations 304 and 306 continues to increase pressureuntil engagement of the first hydraulic clutch 114 is finished. Forexample, when the operator stops depressing accelerator pedal 50, theCPU 40 may determine that the first hydraulic clutch 114 has the correctamount of slippage and the engine 12 is providing the correct amount oftorque to drive axle 34.

The CPU 40 may continue to increase pressure to the first hydraulicclutch 114 in operations 304 and 306 until the first hydraulic clutch114 completely locks input shaft 112 to the first driven gear 116 inoperation 308 and while the drive axle clutches in clutch system 142remain engaged. The vehicle 10 is now moving and the start up sequencefor the vehicle 10 performed by the CPU 40 is completed in operation310.

FIG. 6 is a state diagram further explaining how the transmission system14 shifts between different gears. The example described below showstransitions between three different gears. However, more or fewer thanthree gears can be used in the transmission system 14. Transitionsbetween additional gears would operate similarly to the transitionsbetween the second and third gears as described below. FIG. 6illustrates normal up and down shifting and also shows how the firstgear is torque limited to prevent engine stalling and to preventoverloading the drive axle.

In one embodiment, the operations described in FIG. 6 are controlled bythe CPU 40 previously shown in FIG. 4. Example control valve pressuresare used in FIG. 6 for illustrative purposes but alternative pressurescan be used to provide similar clutch pack modulations. In this example,a 0 pounds per square inch (psi) pressure is associated with acompletely unlocked hydraulic clutch. A 20 psi hydraulic clutch pressureis associated with a touch point where the clutch is just starting totransfer torque to the drive axle 34. A 40 psi pressure represents aclutch that is lightly engaged (slipping) and transfers only a partialamount of torque to reduce impact on the vehicle when a one-way bearingis initially engaged. A 140 psi pressure is associated with a fullylocked hydraulic clutch.

The CPU 40 can determine from the gear ratios currently being used inthe transmission system 14, Engine Rotations Per Minute (ERPM) 30, andTransmission Output Shaft Speed (TOSS) 18 (see FIG. 4) when there iszero slip in a particular hydraulic clutch 114, 118, or 130. Traveldownshift speed values and travel upshift speed values as describedbelow are predetermined variables based on accelerator pedal position 44and ERPM 30.

The vehicle 10 and transmission system 14 are initially in a neutralstate 320. A vehicle move command condition 321 moves the transmissionsystem into a first gear slip state 322. The pressure in the firsthydraulic clutch 114 is decreased if the ERPM is less than apredetermined engine stall speed. Otherwise, the pressure in the firstclutch is increased. Varying the clutch pressure is alternativelyreferred to as modulation.

When the measured clutch slip in condition 323 is zero, the first clutch114 is locked by increasing the clutch pressure to 140 psi. Thetransmission system also moves into a first gear locked state 324. Ifthe ERPMs drop down below a predetermined downshift speed #1 incondition 325, the CPU moves the transmission system back into firstgear slip state 322. The CPU uses a first gear modulation chart inmemory 48 to determine what pressures to then apply to the first clutch114. In this example, the CPU starts at 40 psi to reduce the torque onthe engine 12 and then varies the clutch pressure according to theaccelerator pedal position 44, ERPM 30, and TOSS 18.

Otherwise, the transmission system stays in the first gear locked state324 until the ERPM rises above a predetermined upshift speed #1 incondition 326. When the ERPM rises above the upshift speed #1 value, theCPU moves the transmission system into second gear slip state 327. Inthis example, the CPU starts the pressure in the second hydraulic clutch118 at 20 psi while keeping the first hydraulic clutch 114 in a fullylocked condition. While in the second gear slip state 327, the CPUincreases or modulates the pressure based on mappings of the acceleratorpedal position 44, ERPM 30 and TOSS 18 in the second gear modulationchart.

If the ERPM drops below the down shift speed #1 value in condition 328,the CPU 40 moves the transmission system back into the first gear slipstate 322 and the first gear modulation pressure starts at 40 psi in thefirst gear modulation chart. The clutch pressure is set to 40 psi toquickly move the hydraulic clutch 118 to a beginning initial slippingcondition.

While in second gear slip state 327, the CPU continues to increase thepressure in hydraulic clutch 118 until the second clutch 118 has zeroslip in condition 329. The pressure is then set to 140 psi to solidlyhold the hydraulic clutch 118 in a second gear locked state 340. Thepressure in the first hydraulic clutch 114 is also set to 40 psiallowing the transmission system to quickly respond to any downshiftback to first gear slip state 322.

In second gear locked state 340, a reduction of the ERPM below apredetermined down shift speed #2 value in condition 341 causes the CPUto move back to first gear slip state 322. The pressure in clutch 118 isreduced down to 0 psi and the first clutch 114 is entered at 40 psi inthe first gear modulation chart. The controlled reduction of thepressure in the second clutch 118 down to 0 psi reduces vehicle joltthat could happen if the second gear were instantly disengaged. If theERPM rises above a predetermined upshift speed #2 in condition 342, theCPU moves the transmission system into a third gear slip state 343 andstarts hydraulic clutch 130 at 20 psi in a third gear modulation chart.

The CPU continues to modulate/increase the pressure in the hydraulicclutch 130. If the ERPM drops below a predetermined downshift speed #3value in condition 344, the CPU moves the transmission back to thesecond gear slip state 327 and starts with 40 psi in the second gearmodulation chart. Otherwise, the CPU in third gear slip state 343continues to increase the pressure in the third hydraulic clutch 130until there is zero clutch slip in condition 345. The CPU then sets thepressure in hydraulic clutch 130 to 140 psi and moves into third gearlocked state 346. The second gear pressure is also reduced down to 40psi to provide a quick response to any transmission transitions back tosecond gear slip state 327.

The transmission system stays in the third gear locked state 346 unlessthe ERPM falls below the downshift speed #3 value in condition 347. Inthis case, the transmission system moves back into second gear slipstate 327, the pressure in the hydraulic clutch 130 is modulated down to0 psi, and the CPU starts the second clutch 118 at 40 psi in the secondgear modulation chart.

The system described above provides relatively simple transitionsbetween gears without requiring precise synchronized engagement anddisengagement of different clutches during gear transitions. Clutches donot have to be fully disengaged during a gear transition thereforepartially engaged or disengaged clutches will not create unnecessaryheat and reduce the overall efficiency of the transmission system 14.Additional gears and equivalent modulation states could be included inthe transmission system 14. The different down shift values, upshiftvalues, gear modulation charts, and psi pressures can vary for differenttypes of vehicles and different types of transmission systems.

FIG. 7 is a flow diagram describing in more detail how the CPU 40controls the hydraulic clutches when the vehicle 10 is located on aninclined grade. As mentioned above, the transmission system 14 canoperate with any type of conventional drive axle with directionalcontrol. However, in one embodiment the transmission system 14 mayoperate in conjunction with the clutch system 142 shown in FIG. 1.

A base braking torque is defined in a look up table contained in memory48 (FIG. 4) as the minimal braking torque. The base braking torque valuemay be determined by experimenting with the lowest value that preventsthe vehicle 10 from rolling on a grade with a given slope. The purposeof the minimal fixed torque is to stop the vehicle 10 on flat terrainand prevent or limit rolling on a grade.

When the vehicle 10 is stopped in operation 350, the CPU 40 in operation352 disengages the hydraulic clutches in the transmission system 14 andfully engages all the direction clutches in the clutch system 142 in thedrive axle 34. When the vehicle 10 is commanded to move forward orreverse in operation 354, the CPU 40 slips the first hydraulic clutch114 and keeps the selected clutch engaged in operation 356 and at thesame time decreases and modulates the opposing travel direction clutchesin the clutch system 142 to a minimal value to prevent the vehicle 10from jerking in the selected direction.

The vehicle 10 then starts moving. If the vehicle 10 starts moving inthe opposite direction in operation 358, the CPU 40 increases theengagement of the first hydraulic clutch 114 in operation 360. If thevehicle 10 continues to move in the opposite direction in operation 362,the CPU 40 further increases engagement of the first hydraulic clutch114 in operation 360. When the vehicle 10 starts moving in the selecteddirection and the acceleration pedal position is greater than athreshold value, the CPU 40 fully releases (neutralizes) the opposingdirection clutch(es) in clutch system 142 in operation 366. The CPU 40in operation 368 fully engages the first hydraulic clutch 114 when thevehicle speed indicated by speed and direction signal 18 is greater thana calculated engine stall speed.

Thus, control system shown above controls torque to prevent engine stalland clutch damage due to overheating. Torque control is accomplished byslipping the selected direction clutches in the clutch system 142, or inthe first hydraulic clutch 114. The clutch pressure is derived from acalculated engine stall torque.

The engine 12 will not be reduced below a minimum speed which maintainsenough torque plus a pre-set safety margin to prevent stalling. If theclutch energy exceeds a limit, the torque capacity of the clutch isreduced by reducing clutch pressure or fully disengaging the clutch toprevent damage. Engine speed will be commanded by the CPU 40 to aminimum torque without stalling.

If the clutch energy limit is exceeded, then slipping may alternatebetween the selected direction clutch(es) in clutch system 142 and thefirst hydraulic clutch 114, while maintaining a constant transmittedtorque. A software clutch energy estimator that monitors clutch heat canbe implemented by CPU 40 according to oil temperature, clutch pressure,cooling rate, and slip rate measured via the CPU 40 and the sensors inFIG. 4. When the estimated clutch energy is reduced to an acceptablevalue, then clutch torque can be increased smoothly within thermallimits to fully re-engage normal driving torque and vehicle performance.During subsequent engine braking, the highest third gear hydraulicclutch 130 can be engaged to connect the engine 12 with output shaft126.

The system described above can use dedicated processor systems, microcontrollers, programmable logic devices, or microprocessors that performsome or all of the operations. Some of the operations described abovemay be implemented in software and other operations may be implementedin hardware.

For the sake of convenience, the operations are described as variousinterconnected functional blocks or distinct software modules. This isnot necessary, however, and there may be cases where these functionalblocks or modules are equivalently aggregated into a single logicdevice, program or operation with unclear boundaries. In any event, thefunctional blocks and software modules or features of the flexibleinterface can be implemented by themselves, or in combination with otheroperations in either hardware or software.

Having described and illustrated the principles of the invention in apreferred embodiment thereof, it should be apparent that the inventionmay be modified in arrangement and detail without departing from suchprinciples. We claim all modifications and variation coming within thespirit and scope of the following claims.

1. A vehicle comprising: an engine; a transmission comprising an inputshaft operably connected to the engine for receiving torque, and anoutput shaft operably connected to a drive axle for transferring thetorque to the drive axle; a drive gear connected to either the inputshaft or to the output shaft for rotation therewith; a driven gearconnected to the other of the input shaft or the output shaft forselective rotation therewith via operation of a first clutch such thatthe driven gear engages the drive gear; and a processor configured tooperate the first clutch to modulate transmission of the torque from theinput shaft to the output shaft; wherein the drive axle comprises afirst clutch system operably connected to a first drive wheel; andwherein the processor is further configured to operate a second clutchof the first clutch system to modulate transmission of the torque torotate the drive wheel in a first rotational direction, and to operate athird clutch of the first clutch system to modulate transmission of thetorque to rotate the drive wheel in a second rotational direction thatis opposite the first rotational direction.
 2. A vehicle according toclaim 1, wherein the output shaft is configured to rotate in a singledirection.
 3. A vehicle according to claim 1, wherein the input shaft isdirectly connected to the engine without a torque converter interposedbetween the transmission and the engine.
 4. A vehicle according to claim1, wherein the processor is further configured to operate the firstclutch to inhibit transmission of the torque from the input shaft to theoutput shaft while operating the second clutch and the third clutch tobrake the drive wheel.
 5. A vehicle according to claim 4, wherein, inresponse to an input indicating that the vehicle should move from a stopin either a selected forward or a selected reverse direction, theprocessor is further configured to operate the first clutch to modulatetransmission of the torque from the input shaft to the output shaftwhile operating the second clutch and the third clutch to modulatetransmission of the torque to the drive wheel such that the vehiclemoves in the selected forward or reverse direction.
 6. A vehicleaccording to claim 1, wherein the processor is further configured toslip the second clutch and the third clutch such that torque resistanceat the drive wheel does not stall the engine.
 7. A vehicle according toclaim 6, wherein the processor is further configured to slip the firstclutch such that torque resistance at the drive wheel does not stall theengine; and wherein the processor is further configured to alternatebetween slipping the first clutch and the clutch system of the driveaxle such that a clutch energy limit is not exceeded by any one of thefirst, second, or third clutch while torque is transmitted to the drivewheel.
 8. A method of operating a vehicle comprising: receiving avehicle stop command at a processor; via the processor, disengagingclutches in a transmission such that engine torque is not transmitted toa drive axle in response to receiving the vehicle stop command; and viathe processor, engaging clutches in the drive axle such that a wheelconnected to the drive axle is inhibited from rolling in response toreceiving the vehicle stop command.
 9. A method of operating a vehicleaccording to claim 8, further comprising: receiving a command to movethe stopped vehicle in a forward direction at the processor; via theprocessor, slipping one clutch in the transmission such that enginetorque is transmitted to the drive axle in response to receiving thecommand to move the stopped vehicle forward; via the processor,retaining a clutch in the drive axle associated with the forwarddirection engaged in response to receiving the command to move thestopped vehicle forward; and via the processor, slipping a clutch in thedrive axle associated with a rearward direction such that the vehiclemoves in the forward direction without stalling an engine in response toreceiving the command to move the stopped vehicle forward.
 10. A methodof operating a vehicle according to claim 8, further comprising:receiving a command to move the stopped vehicle in a rearward directionat the processor; via the processor, slipping one clutch in thetransmission such that engine torque is transmitted to the drive axle inresponse to receiving the command to move the stopped vehicle rearward;via the processor, retaining a clutch in the drive axle associated withthe rearward direction engaged in response to receiving the command tomove the stopped vehicle rearward; and via the processor, slipping aclutch in the drive axle associated with a forward direction such thatthe vehicle moves in the rearward direction without stalling an enginein response to receiving the command to move the stopped vehiclerearward.